Sealing or packaging of semiconductor chips is usually carried out by transfer molding of a sealing compound. In some detail, a semiconductor chip is set in a mold cavity, and a thermosetting resin tablet preheated usually by radiofrequency induction heating is put in a transfer pot. The tablet is plasticized by heating and, at the same time, pressed by a plunger and introduced into the cavity through a sprue, a runner, and a gate to complete shaping and curing.
Resin tablets for this use have conventionally been produced by cold compression molding comprising plasticizing and kneading raw materials in a roll or an extruder, cooling the resin compound, pulverizing the solidified resin compound, feeding a prescribed given amount of the powder to a mold, and compressing the powder into a tablet between an upper plunger and a lower plunge at room temperature. Tablets obtained by this plunger-plunger compression molding suffer from considerable incorporation of metallic impurities due to the metal wear during pulverization, which has made it difficult to obtain high purity tablets. Besides, the tablets suffer from noticeable void formation.
While moldability of conventional resin tablets for sealing of semiconductors essentially depends on physical properties of the resin used, it is largely influenced by impurities in the tablets, particularly metallic impurities. The quantity of heat generated by radiofrequency induction heating during preheating of tablets is expressed by .omega.V.sup.2 .epsilon. tan .delta., wherein .omega. is a frequency; V is an applied voltage; and .epsilon. is a dielectric constant of the tablet. Because dielectric constant .epsilon. of resins ranges from 4 to 5 while that of metals is substantially unlimited, the parts where metallic impurities are present undergo abnormal heating and thereby excessive curing. In some cases, sparks occur in such parts to cause excess current to pass through the induction heater, which often causes troubles of the induction heater. Even where transfer molding is not preceded by radiofrequency induction heating, semiconductor devices sealed with a resin compound containing metallic impurities are inferior in reliability. Therefore, resin tablets containing metallic impurities are unfavorable for use as a semiconductor sealing compound.
Further, conventional tablets have many voids. Since .epsilon. of air in the voids is smaller than that of the resin, heating unevenness occurs due to the voids. Since a resin viscosity (.eta.) logarithmically changes with resin temperature (T) as is seen from equation: .eta.=10.sup.(A+kT), the above-described heating unevenness develops unevenness of fluidity of the sealing resin during transfer molding, likely causing defective molding.
On the other hand, sheet-extrusion has been proposed for production of tablets for semiconductor sealing, in which raw resin materials are plasticized and kneaded in an extruder, and the extruded sheet is rolled and then punched out or cut to obtain tablets. According to this process, too, incorporation of metallic impurities inescapably occurs during the punching or cutting operation. Also, the resulting tablets contain voids generated during kneading and therefore have a compressibility (hereinafter defined) of about 90% at the highest. Further, the extrusion technique has difficulty in obtaining a thick sheet, resulting in a failure of obtaining tablets with a high L/D ratio (hereinafter defined). Furthermore, tablets obtained by punching have poor precision in shape and weight. Thus, the sheet-extrusion is unsuited for production of resin tablets for semiconductor sealing as desired.
It has been suggested to cast a molten resin composition into a metallic mold followed by tableting. However, generally having a high viscosity, the resin composition used tends to form large voids. The large voids have great influences particularly in the production of tablets having a high L/D ratio.
A molding method comprising filling a molten resin composition into a mold with an injection pressure applied, followed by tableting has been attempted. However, this method encounters with extreme difficulty in obtaining tablets having a compressibility of 98% or higher because of such a low pressure applied as injection pressure. There is found no report of using the tablet obtained under pressure on the level of injection pressure for semiconductor sealing.